CONCLUSIONS 105 
pieces under a very slight stimulus. It has often thus 
been represented. But this, after all, is a static view. 
We have discovered this very fundamental fact, that 
the resting metabolism goes faster when the tissue is 
more abounding in life and is more irritable. It is the 
burning substance which is irritable, or, perhaps, the 
carbon dioxide thus formed conditions in some way 
the vital or irritable reaction. It requires the expendi- 
ture of energy to keep living matter in an irritable state. 
The reaction is dynamic and not static. Living matter 
has been conceived by many physiologists (of whom I 
may mention only one of the leading exponents, Verworn, 
for the various modifications of this view of individual 
authors are not fundamental modifications) as being 
composed of very complex unstable molecules, or aggre- 
gates of molecules which are very unstable. These 
are called biogens. Now this view is essentially static. 
There is no reason why a biogen should not be isolated 
if our methods were but fine enough. There is no reason 
why a collection of biogens should not exist without any 
metabolism; why, in other words, suspended anima- 
tion should not be possible. But the facts which we 
have discovered of the parallelism of the production 
of carbon dioxide and irritability lend support, it would 
seem, to the dynamic, rather than to the static, view. 
We can picture the process, perhaps, in the following 
crude and, of course, indefinite manner: The life- 
process may be considered as a bicycle in motion. The 
living process is an unstable condition. It is like a 
chemical system, the system as a whole having a certain 
stability, but being at the same time the seat of intense 
chemical change. Itis in an unstable equilibrium. The 
